High-lying long-lived doubly excited states near ionizatin thresholds of helium

Long-lived, excited neutral particles, arising from the interaction of monochromatic synchrotron radiation with ground state He atoms, have been observed at photon energies close to the He+(N = 1,2,3 and 4) ionization thresholds. The measurements have been made using an unconventional experimental arrangement in which charged particles, responsible for the dominant signal in conventional photon impact studies, are prevented from reaching the detector. For N ≥ 2, it appears that the formation, via photoexcitation, of relatively long-lived, doubly excited Rydberg states is a necessary step in the production of the observed signal. Four processes that might account for the production of the excited neutral particles are considered. The simplest, that atoms in the initial doubly excited Rydberg states are themselves directly observable, is considered unlikely. However, the lifetimes of the photoexcited double Rydberg states may be increased through the effects of electric fields present in the apparatus. Transitions from these double Rydberg states, occurring either as a result of collisions or by fluorescence, could result in metastable, singly excited neutral particles. The presence of signal related to the existence of long-lived doubly excited states could be significant in the interpretation of other photon impact measurements. (Author)